Instrumentation for use in spectroscopy of charged particles makes use of electrons or ions which are emitted from a substance after being bombarded or irradiated with primary electrons or ions from a source such as an electron gun. One charged particle spectroscopy technique is known as Auger electron spectroscopy. In this technique, a target sample material is placed in an ultra high vacuum (UHV) environment, typically about 10−10 Torr to 10−9 Torr, and upon being bombarded with primary electrons from some source, such as an electron gun, the sample gives off a variety of emissions. Among these are X-rays, secondary electrons, and reflected primary electrons from the source. The emissions include Auger electrons (a particular class of secondary electrons) that are emitted in a manner well known in the art. The energy spectra of these Auger electrons may be analyzed to determine information about chemical species present at the surface of the target. Auger electron spectroscopy typically requires a primary electron energy over voltage that is about 1.5 times an optimum required voltage. This energy is typically sufficiently low that the primary electrons do not damage the sample's surface. Auger electron spectroscopy is useful as a surface analytical technique because the energies of the electrons emitted are typically in the range of 50 eV to 3 KeV, and at this energy they cannot escape from more than a few nanometers deep in the surface (of course, the higher the energy, the thicker the layer from which they can escape).
In the art of Auger electron spectroscopy, electron energy analyzers operate by injecting the diverging electrons into an electric field using a few simply shaped electrodes. Auger electrons injected from the sample into the electric field are deflected by the field such that electrons of a predetermined energy are brought to a focus. By positioning a collector apparatus at this focus, Auger electrons of a predetermined energy may be selected and detected. In some electron energy analyzer designs the voltage impressed across the electrodes may be swept through a range of values and a collector signal may be detected as a function of these applied potentials as the electrons are collected. By plotting or otherwise analyzing the signal (or a derivative thereof) as a function of applied potential, an energy spectrum of the injected electrons may be determined.
Auger electron spectrometers conventionally use a cylindrical mirror analyzer to obtain a secondary electron energy spectrum. A cylindrical mirror analyzer uses electric field between two concentric metal cylinders to select secondary electrons according to energy. Only electrons with the right energy will make it through the field region between the cylinders and strike a detector. A spectrum is obtained by varying the voltage applied between the cylinders and measuring the electron signal as a function of energy.
Auger spectroscopy is often limited by contamination of the sample surface being analyzed. Contaminants, such as hydrocarbons, often collect proximate a location where an electron beam strikes the sample's surface. Such contaminants tend to preferentially migrate or diffuse to the region under electron bombardment. The accumulation of surface contaminants under electron beam scanning can cause a problem for Auger analysis because the Auger electron signal from the contaminants can grow over time to the point that it is larger than the actual target sample. This can limit the useful amount of time for making an Auger measurement.
It is within this context that embodiments of the present invention arise.